POLICE have foiled a major plot to "commit mass murder on an unimaginable scale" by destroying up to nine passenger jets in mid-air.

The targets were flights leaving UK airports to the US.

Anti-terror police arrested 21 people in London, the Thames Valley and Birmingham last night as stringent new security measures were imposed on all UK airports.

A ban on hand luggage along with other checks and precautions led to chaos at airports where scores of flights were cancelled and massive queues built up.

Metropolitan Police Deputy Commissioner Paul Stephenson told reporters at New Scotland Yard that the plot was designed to cause "untold death and destruction".

He added: "This was intended to be mass murder on an unimaginable scale."

Searches were on-going at a number of addresses, he added.

Sources indicated that the majority if not all of those arrested were British.

Senior sources said the terrorists were believed to be planning to blow up as many as nine aircraft in a simultaneous attack.

The majority of those arrested were understood to be of Pakistani origin.

Home Secretary John Reid said the plot represented "a major threat to the UK and international partners".

He said that the terrorists planned to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life".

The official UK terror threat state was raised to 'critical' - its highest level - as travellers were urged not to attempt to catch flights unless it was essential.

Among the measures introduced at airports were specific restrictions on taking liquids on board, prompting speculation that the terrorists were planning to smuggle liquid explosive devices on to jets.

If it had been successful, the terror plot would have been the first 'spectacular' since last year's July 7 attacks on London, which killed 52 people.

It is believed the covert investigation had been going on for several months and it is understood that officers made the arrests overnight not because they feared an attack was likely to happen today but for other intelligence reasons, which meant they had to act quickly.

The Department for Transport said it was hoped that the extra security measures would be needed "for a limited period only".

But former Metropolitan Police Commissioner Lord Stevens said such drastic steps would not have been taken unless there was an "absolute need".

"You know there is going to be a fair amount of disruption and chaos and that is balanced against trying to keep things as normal as possible.

"But they will not have done anything unless there was an absolute need for it."

Only yesterday Mr Reid warned that Britain was facing its most sustained period of serious threat since the end of the Second World War.

In a high-profile speech on security, he warned that the security services and police alone could not guarantee 100% success in combating terrorism.

There were also claims yesterday that the police and MI5 had foiled at least 13 suspected terrorist plots in Britain in the last six years.

It was reported that MI5 had switched more resources to counter terrorism and that 87% of its budget was now spent on combating the threat.

Downing Street said today's anti-terror operation had been undertaken "with the full support" of Prime Minister Tony Blair, who is on holiday in the Caribbean.

He had been "in constant contact" with the situation in the UK and had briefed US President George Bush overnight, a spokesman said.

Deputy Prime Minister John Prescott - in charge of the day-to-day running of the country in the absence of Mr Blair - was "in constant contact" with Mr Reid, his spokeswoman said.

The US government also raised its threat assessment to the highest level for commercial flights from Britain to America.

Homeland Security Secretary Michael Chertoff said: "We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted."